The alloy prepared from powder materials by press sintering has a compacted structure, and various powder alloy materials can be used in combination in the form of a dispersion for preparing such alloy. Because of these advantages, the alloys of this type can be expected to have higher strength and higher toughness than those obtained by the melting process. The press sintering processes are therefore thought to be useful for developing new alloys. Similarly, new ceramics having high toughness are produced from powder ceramic materials also by press sintering.
The press sintering processes useful for this purpose include, for example, the powder vehicle process and molten bath process.
The powder vehicle process is described, for example, in "Nikkei Mechanical," p. 128, July 2, 1984, published by Nikkei-McGraw-Hill Co., Ltd. According to the publication, a green compact is embedded in a pressure medium powder which is not reactive with the compact and which is placed in a tubular mold having a bore extending centrally therethrough. The tubular mold is enclosed in a tubular heat insulator, which is provided with a heating induction coil on its outer periphery. A pair of press rods is inserted into the central bore of the tubular mold from its opposite ends, whereupon the green compact is heated by the induction coil. Consequently, the green mold is sintered while being subjected to pressure through the pressure medium powder.
However, the powder vehicle process has the following drawbacks.
(1) It is difficult to isostatically press the green compact because the pressure medium is a powder.
(2) The mold, which is to be heated from outside, needs to be heated to a temperature higher than the sintering temperature. Consequently, the mold must have an increased wall thickness and is difficult to design.
(3) The mold, which is adapted to be heated from outside, requires a prolonged period of time when to be heated to the specified temperature and is therefore low in productivity.
The publication, p. 129, also discloses the molten bath process which is free of the above drawbacks. With this process, a green compact is immersed in molten glass serving as a pressure medium and placed in a crucible. The crucible is placed into a pressure-resistant container having an open upper end, a bottom and an inside heater, and the open upper end is closed with a cap having a high-pressure gas inlet. The green compact is sintered as desired by being heated with the heater while being subjected through the molten glass to the pressure of the gas admitted through the cap inlet.
Nevertheless, the molten bath process has the following drawbacks.
(4) It is difficult to obtain a sufficient pressure because a gas is used as the pressure source, while the cap is difficult to seal off completely.
(5) Even if the clearance between the container and the cap can be sealed off completely, a gas pressure of thousands of atmospheres, when needed, requires the operation of a large compressor for a prolonged period of time. Along with the drawback (4), this drawback results in low productivity and an increased production cost.